1. Field of the Invention
The present invention relates generally to FM laser radar and, more particularly, to an elegantly simple, high-voltage, electro-optic "EO" crystal driver for use in such radars.
2. Description of Related Art
Laser radar systems, employing an intensely focused beam of light to detect the presence, position, and motion of objects, have been used in many applications, especially in the radar, communications, and measurement fields. Militarily, these systems have been implemented in conjunction with new cruise missile and tactical fighter technology wherein laser radar has provided obstacle avoidance, as well as terrain following functions. Laser radar systems have also enabled sophisticated target homing capabilities for accurately guiding a missile or plane toward a target by using a distinguishing feature of that target.
Pulse compression is used advantageously in radar systems, including laser radar or "lidar" systems which operate at optical rather than radio frequencies. A lidar system employing linear FM pulse compression is disclosed in U.S. Pat. No. 4,666,295, entitled "LINEAR FM CHIRP LASER," issued May 19, 1987 to R. Duvall et al.
Pulse compression involves the transmission of a long coded pulse and the processing of the received echo to obtain a relatively narrow pulse. The increased detection capability of a long-pulse radar system is achieved while retaining the range resolution capability of a narrow-pulse system. Transmission of long pulses permits a more efficient use of the average power capability of the radar without generating high peak power signals. The average power of the radar may be increased without increasing the pulse repetition frequency (PRF) and thereby decreasing the unambiguous range of the radar.
In pulse compression radar, a long pulse is generated from a narrow pulse which contains a large number of frequency components with a precise phase relationship between them. The relative phases are changed by a phase-distorting filter such that the frequency components combine to produce a stretched, or expanded pulse which is then amplified and transmitted. In some classes of radar where the transmitted signal frequency is much greater than practical phase-distorting filters such as surface acoustic wave (SAW) devices can accommodate, the transmitter must be frequency modulated directly in order to produce an expanded pulse. This is the case for laser radar. The received echo is processed in the receiver by a compression filter, which readjusts the relative phases of the frequency components so that a narrow or compressed pulse is again produced.
Various pulse compression methods are known in the art, including linear frequency modulation (FM), nonlinear FM, and phase-coding. Linear FM pulse compression, also known as "chirp," is especially advantageous in that, in addition to determining the range to a target, the relative doppler can be obtained simultaneously with resolution equivalent to that of long-pulse radar systems.
FM "chirped" laser radar which involves heterodyne or coherent detection has proven to be particularly useful in these applications. Typically in these systems, a continuous wave (CW) transmitter emits laser light at a preselected center frequency. This emitted light is frequency modulated into linear "chirps" by passing it through an electro-optical device disposed within the cavity of the transmitter. The frequency variation created is preferably linear, and the frequency-versus-time characteristic of the signal typically has a trapezoid pattern.
The "chirped" signal is directed toward a target and then reflected back therefrom, creating a "return" signal associated with that target. The time taken by the transmitted signal to reach its target and then return causes the return signal to be displaced in time with respect to the transmitted signal.
The instantaneous frequency difference between the transmit and return signals may be obtained by comparing the return signal to a reference signal, which is typically a sample of the transmitted signal created by retaining a portion of the transmitted beam using a beam splitter. Properly scaled, this instantaneous frequency difference can be used to, in effect, demodulate the return signal in order to obtain information about the target.
As noted above, FM laser radar utilizes an electro-optical device, typically an EO (electro-optic) crystal to modulate the frequency of the laser transmitter. The EO crystal is driven by a high-voltage driver, and the shift in frequency imparted by modulation is proportional to the electric field across the crystal. Linear FM pulse compression radar requires the frequency modulation to be linear to much less than 1%, which, in turn, requires a voltage ramp across the EO crystal that is linear to less than 1%.
The bandwidth of the high-voltage driver, i.e., the driver's ability to respond quickly, is also proportional to the overall linearity achievable. For a trapezoidal frequency modulation waveform, the bandwidth requirement can exceed ten times the pulse repetition frequency "PRF" in order to achieve 1% linearities.
For some applications, the linearity requirements are much stricter. To meet such strict requirements, the outgoing linear FM pulse must be monitored for linearity, and an error signal must be generated which can be used to make fine corrections to the EO crystal voltage ramp. To permit fine corrections to be made, the high-voltage driver must be able to respond within a small fraction of the total trapezoidal pulse width. In such case, the driver bandwidth must be on the order of a hundred times the PRF.
Previously-known high-voltage drivers have required their high-voltage components to operate in their linear regions as part of a control loop. Therefore, in terms of bandwidth, the performance-limiting components usually are the high-voltage elements and/or the step-up transformer used to increase the output voltage of the high-voltage driver. In a typical configuration, the control loop bandwidth of the high-voltage driver (amplifier) is typically less than 10% of the lowest speed component in order to meet loop stability requirements. Therefore, the time constant or speed at which linearity corrections can be made is 100 times less than the lowest speed component.